The present invention relates to a semiconductor device including a device region formed in an epitaxial layer, and more particularly to a semiconductor device formed of a material, such as SiC, which is liable to cause surface scattering.
Conventionally, MOS transistors are generally used in semiconductor devices. In particular, Si-based MOS transistors are used in various types of semiconductor devices.
There is a demand for an MOS transistor, wherein the withstand voltage is high and the base is thin, to be used in semiconductor devices having a high withstand voltage. As an MOS transistor which meets the demand, a SiC-based MOS transistor is expected, since the thickness of a SiC-based MOS transistor for a given voltage can be about 1/10 of that of a Si-based MOS transistor.
However, this type of MOS transistor has the following drawback: the mobility of carriers in a channel region (channel mobility) is much lower than that in the bulk SiC, resulting in high ON resistance.
The cause of the drawback is as follows. Since SiC cannot be easily polished by chemical mechanical polishing (CMP), great roughness remains on the surface of SiC after polishing. For this reason, when a gate oxide film is formed on the surface of SiC polished to a predetermined thickness, the channel mobility is reduced by surface-roughness scattering on the interface between the gate oxide film and SiC. Further, a number of dangling bonds due to the lattice structure exist on the surface of SiC, so that the channel mobility is reduced by coulomb scattering on the MOS interface. As a result, the channel mobility is very low, and the ON resistance is high.
In general, to grow an epitaxial layer, the semiconductor substrate is polished so that the substrate is misoriented .theta. degree from the low index direction axis, and an epitaxial layer is formed on the polished off-axis surface of the substrate. This is because a monocrystalline layer including few crystal defects can be obtained by forming a step structure on the surface of a substrate and epitaxially growing monocrystal on the step surface. However, it is known that the greater the off-axis angle .theta., the lower the effective channel mobility due to scattering caused by the steps, and the greater the ON resistance.
If the substrate is made of Si, since the off-axis angle is generally set smaller than 1.degree., the number of steps is small. Hence, the channel mobility is not practically reduced. However, in the case of SiC, the off-axis angle is set to about 4.degree. for step-controlled epitaxy and the steps are steep due to a characteristic periodical structure of the hexagonal system. As a result, the step density becomes large and thus considerable scattering occurs. Thus, the effective channel mobility is reduced to an extent which cannot practically be ignored, and the ON resistance is increased.
Further, if the surface of the epitaxial layer makes an angle with a crystal surface along a low index number direction macroscopically, and if a trench is formed without taking the direction thereof into consideration, the side wall of the trench is inevitably deviated from the crystal surface along the low index number direction. As a result, the lattice structure on the side wall is disordered and a number of dangling bonds are generated. For this reason, when a current path is formed on the side wall of the trench, the following problem is raised: the surface scattering becomes greater due to surface-roughness scattering and coulomb scattering, and the ON resistance is increased.